As an antenna for a wireless device that receives high-frequency electrical waves, one using earphones themselves, one formed by arranging conductor lines within a housing of the wireless device, and the like have typically been used. It has been difficult to obtain a sufficient receiving sensitivity with these antennas. Thus, a loop antenna introduced in Japanese Patent Application Publication No. 10-84209 has been developed. Antennas of this type have a configuration in which a loop antenna is attached to a neck strap of a small-sized wireless device, and in which an inductance element is connected in parallel to the loop antenna in such an orientation that a plane of its aperture faces perpendicularly to the surface of the human body.
However, giving no consideration to earphones, and these antennas have addressed an issue that signal lines to the earphones must be provided separately. Thus, an earphone antenna, disclosed in Japanese Patent Application Publication No. 6-22331, has been developed, for example, in which signal lines to earphones are assembled with an antenna for integration into a helmet.
The antenna of the type introduced by, e.g., Japanese Patent Application Publication No. 10-84209 mentioned above and the like, has addressed the issue, first of all, that no consideration is given to the integration of earphones thereinto.
Further, the earphone antenna in which audio signal lines to earphones are integrated with an antenna has addressed the issue that the human body exerts serious influence upon a wireless device through the antenna due to the earphones directly touching the human body and the stability of reception is greatly compromised. Furthermore, the fact that a matching section of the antenna and an equipment section, such as a tuner, are placed within the same equipment has imposed another problem that the earphone antenna is susceptible to noises of the equipment.
While this problem occurs also on radios, this trend is observed more notably on, e.g., portable liquid-crystal TVs (television receivers) that receive television broadcasting waves having receiving frequencies higher than their broadcasting waves. However, the fact is that no effective measures have been taken for that problem.
Furthermore, where a television broadcast is to be received, there has been still another problem that, for a wide band of television broadcasting frequencies, it is difficult to ensure a sufficient receiving sensitivity over the entire wide band.
That is, so-called portable liquid-crystal TVs (television receivers) need to receive high-frequency signals over a very wide band of 90 to 770 MHz. Specifically, in a case of a high-frequency band usable for television broadcasting in Japan, usable frequencies include 90-108 MHz (1-3 channels), 170-220 MHz (4-12 channels) in the VHF band, and 470-770 MHz (13-62 channels) in the UHF band.
It is difficult to obtain a sufficiently high receiving sensitivity over such a wide frequency band, and thus it has been unavoidable that a low receiving sensitivity is exhibited over a certain frequency band. The reason is that it is the loop length of the loop antenna that defines a receiving frequency band, and the receiving sensitivity of the loop antenna decreases for frequencies outside that frequency band.
For this reason, attempts have also been made to develop a sensitivity adjusting means. For example, a sensitivity adjusting component into which a magnetic substance is movably inserted is provided to adjust the amount of insertion of the magnetic substance in accordance with a frequency for reception. However, according to such a sensitivity adjusting means, there has been an inconvenience that sensitivity adjustment must be made every time the frequency for reception is switched.
Note that, as to digital terrestrial broadcasting planned to be introduced in the future, which uses only the UHF frequencies as its broadcasting electrical waves, in a case of a receiver ready for the digital terrestrial waves, it can be said that the receiving frequency band is narrower than in a case of a receiver ready for analog terrestrial waves. However, the conventional earphone antennas are not suitable for receivers ready for digital terrestrial broadcasting. The reason is that, as mentioned above, no effective measures have been taken in order to eliminate high frequency-derived negative influence exerted upon the receivers from the human body via the earphones and the earphone antenna.
The present invention has been made in order to overcome such problems, and an object thereof is to provide an earphone antenna which can eliminate high frequency-derived negative influence exerted upon a receiver side from the human body via earphones, and further, which can remove the influence of noises of the equipment by isolating an antenna section from the receiving equipment, ensure a receiving sensitivity required for a wide band of frequencies without involving a sensitivity adjusting operation, and also transmit audio signals of a television receiver to earphone units, a composite coil and a coaxial cable used therefore, and a wireless device provided with the earphone antenna.